WP had originally intended to replace their secondary passenger service with service only between Salt Lake City and Stockton, allowing a single RDC to make the run. The ICC insisted that WP continue secondary service to Oakland, and WP had to order two RDCs resulting in the meet between them.

Not long after RDCs were being placed in service, there was a bad wreck in the northeat where a team powered train rear ended an RDC. It turns out that the RDC does not reliably shunt signal circuits. The reason for calling the dispatcher would have been to confirm you were in the siding and clear of the main so he could line the signal for the opposing traffic. Somewhere in the WP's rulebook, special instruction or dispatcher's rules of the era should be the requirement to establish an absolute block to protect an RDC on the main in CTC, also this would apply to a Sperry car not under track and time.

This requirement would not apply to freights, since they should reliably shunt the track circuits and not give a false clear. WP might have had a special instruction requiring crews to always check in with the dispatcher at remote sidings before radio service became available.

I recently saw somebody's recent timetable special instructions that had the absolute block provision if the total count of engines and cars was three or less.